Determination of Dissolved Argon and Nitrogen in Water by Direct Aqueous Injection GC-HID

Abstract

A gas chromatography-based automated method was developed for direct aqueous injection analysis of trace gases dissolved in water samples to support studies on air-water exchange processes of chemicals. Initial efforts were focused on the determination of dissolved gases such as Ar and N, because of their potential to serve as “tracer” species (or as “surrogates”) for the air-water exchange process. Direct injection of water samples eliminated time-consuming sample preparation procedures and enabled short analysis cycles. The method employed a GC equipped with a helium ionization detector (HID) to achieve sensitivity sufficient for water analysis with direct injection of 10 μL water via a liquid sample valve. Analytes were isolated from the water matrix using a column switching technique prior to the separation and detection. Chromatographic separation of Ar, O₂ and N₂ was achieved with a long, 30-foot molecular sieve column. However, a short. 6-foot column combined with a chemical scrubber for O₂ was selected in order to ensure accurate quantitation of Ar and shorten the analysis cycle to 15 minutes. The precision for determination of Ar and N₂ was 1% RSD, with a method detection limit of ca. 30 μg/L Ar or N₂ in water and a linear range of ca. 2.5 orders of magnitude.

Key Words:

• Aqueous injection
• GC-HID
• column switching
• dissolved gases